Ten years ago, scientists watching the skies experienced a Halloween fright of cosmic proportions, when space weather degraded GPS signals, affecting land and ocean surveys, and commercial and military aircraft navigation.
The most extreme of what became known as the Halloween Storms hit on October 30, 2003 — ten years ago today. According to the National Oceanic and Atmospheric Agency, the Earth could experience a repeat performance this Halloween, with a 35 percent chance of a major storm at high latitudes.
The U.S. Geological Survey describes the cause of the 2003 storms:
In mid-October 2003, a bundle of concentrated magnetic energy emerged from the Sun’s interior, forming a large sunspot, a site of seething activity. Enormous solar flares soon followed.
Then, on October 28, the sunspot abruptly ejected a concentrated mass of electrically conducting solar wind, flinging it out into interplanetary space toward the Earth. Less than a day later, on October 29, a geomagnetic storm was initiated as the solar wind disrupted the Earth’s protective magnetosphere.
Over the next three days, the “Halloween magnetic storm” would evolve and grow to become one of the largest such storms in half a century. Magnetic storms are global phenomena, and their effects can be easily seen around the world. During the Halloween storm, for example, magnetic direction in Alaska quickly changed by more than 20 degrees. In other words, the storm was so large that it could be measured with a simple compass. The Halloween magnetic storm also produced spectacular aurora, with green phantom “northern lights” seen as far south as Texas and Florida.
“The aurora was exciting,” said Richard Langley, GPS World’s Innovation editor. “I’ve never seen a better one since.”
Langley explained the effect of the phenomenon in his introduction to the October 2004 Innovation article, “Combating the Perfect Storm: Improving Marine Differential GPS Accuracy with a Wide-Area Network.”
It was previously thought that the mid-latitude North American ionosphere was reasonably benign, with minimal storm effects of relevance for marine DGPS users. However, during ionospheric storms in May and October, 2003, [single-frequency] marine DGPS horizontal position accuracies were degraded by factors of 10–30.These degraded accuracies persisted for hours and were well beyond system tolerances specified for marine DGPS users. Such ionospheric activity is not unusual during the years following solar maximum, and is expected to persist for several years.
Langley provides background on what scientists learned from the Halloween Storms in his February 2011 Innovation column, “GNSS and the Ionosphere: What’s in Store for the Next Solar Maximum?”:
The current solar cycle is referred to as cycle 24. During the last solar cycle, cycle 23, the GNSS community was alert and aware of what could happen, and therefore many events were observed and analyzed. Among the most well-known events is a sequence of storms during October and November 2003, commonly referred to as the Halloween Storms.
The most extreme was the storm on October 30, 2003, which resulted from a CME on October 29 at 20:49 UTC, which subsequently impacted Earth’s magnetic field at 16:20 UTC on October 30 and produced a great geomagnetic storm, which lasted for many hours.
Effects on GPS positioning of this storm have been documented by the GNSS research group of the Royal Observatory of Belgium, where kinematic analyses of data from 36 GNSS stations in Europe showed position errors of more than 10 centimeters in the horizontal and up to 26 centimeters in the vertical between 21:00 and 22:00 UTC on October 30. The position errors were largest for locations in northern Europe including Sweden and Norway. The data analysis was carried out using high-quality carrier-phase data, and the processing was based on using an ionosphere-free linear combination of observations from the L1 and L2 frequencies, whereby the first-order effect of the ionosphere is removed from the results. The position errors are thus caused by mainly higher order ionospheric effects.
For navigation-grade GPS positioning, a U.S. National Atmospheric and Oceanic Administration technical memorandum reported that the Wide Area Augmentation System (WAAS) vertical error limit of 50 meters was exceeded for a period of about 11 hours on October 30, 2003. This means that, in practice, WAAS was not available for precision aircraft approaches during that time. The European Geostationary Navigation Overlay Service (EGNOS) was not transmitting during the storm, but simulations carried out later by ESA showed that the boundary regions of the EGNOS coverage area would have been especially affected by a reduction in service availability of about 20–60 percent during that day.
The simulations also showed, however, that in the center of the EGNOS coverage area (in the vicinity of northern Italy), the effect would have been much smaller with a reduction in service availability of only 5–6 percent over the day.
Such large storms are also often accompanied by displays of aurora (aurora borealis and aurora australis) at lower latitudes than normal.
Other Innovation columns assessing the ionosphere’s effect on GPS include: